Decoder and decoding method for lc3 concealment including full frame loss concealment and partial frame loss concealment

ABSTRACT

FIG.  1  illustrates a decoder for decoding a current frame to reconstruct an audio signal according to an embodiment. The audio signal is encoded within the current frame. The current frame includes a current bitstream payload. The current bitstream payload includes a plurality of payload bits. The plurality of payload bits encodes a plurality of spectral lines of a spectrum of the audio signal. Each of the payload bits exhibits a position within the current bitstream payload. The decoder includes a decoding module and an output interface. The decoding module is configured to reconstruct the audio signal. The output interface is configured to output the audio signal.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of copending InternationalApplication No. PCT/EP2020/053620, filed Feb. 12, 2020, which isincorporated herein by reference in its entirety, and additionallyclaims priority from European Applications Nos. EP 19156997.9, filedFeb. 13, 2019, EP 19157036.5, filed Feb. 13, 2019, EP 19157042.3, filedFeb. 13, 2019 and EP 19157047.2, filed Feb. 13, 2019 as well as fromInternational Application Nos. PCT/EP2019/065172, filed Jun. 11, 2019,PCT/EP2019/065205, filed Jun. 11, 2019 and PCT/EP2019/065209, filed Jun.11, 2019, all of which are incorporated herein by reference in theirentirety

The present invention relates to a decoder and a decoding method for LC3frame loss concealment including full frame loss concealment and partialframe loss concealment.

BACKGROUND OF THE INVENTION

Transform based audio codecs rely on coded representations of spectra ofaudio frames. Such spectra consist of a plurality of spectral lines. Dueto various reasons, either some or even all spectral lines may not beavailable on a decoder side. Audio error concealment concepts in thefrequency domain may, e.g., provide means to mitigate artefacts causedby such missing spectral lines. A common approach is, to find as good aspossible replacements for them.

In the prior art, various frame loss concealment techniques areavailable.

Frame loss concealment concepts in the frequency domain are, e.g.,discussed in [1], where, in particular, muting, repetition, noisesubstitution and prediction are mentioned. Those techniques are alwayscombined with a fade-out process, which fades the signal—usually overseveral lost frames—towards either zero or towards some sort ofbackground noise/comfort noise.

In [2], different attenuation factors for frequency bands are proposeddepending on the energy in those bands: A larger attenuation factor may,e.g., be applied for bands with an energy higher than a threshold, and asmaller attenuation factor might be applied for bands with an energybelow that threshold. Moreover, in [2], the energy progression over thelast good frames is observed, and a stronger attenuation is applied, ifthe energy in the last good frame was smaller than in the last but onegood frame.

Furthermore, the spectral shape of the signal might also be fadedtowards some sort of common shape. This approach is used in particularin linear predictive coding (LPC) based codecs, e.g. EVS (enhanced voiceservices), where the LPC coefficients are blended towards some providedmean coefficients.

SUMMARY

An embodiment may have a decoder for decoding a current frame toreconstruct an audio signal, wherein the audio signal is encoded withinthe current frame, wherein the current frame includes a currentbitstream payload, wherein the current bitstream payload includes aplurality of payload bits, wherein the plurality of payload bits encodesa plurality of spectral lines of a spectrum of the audio signal, whereineach of the payload bits exhibits a position within the currentbitstream payload, the decoder having: a decoding module configured toreconstruct the audio signal, and an output interface configured tooutput the audio signal, wherein the decoding module includes an errorconcealment mode, wherein, if the decoding module is in said errorconcealment mode, the decoding module is configured to reconstruct theaudio signal by conducting error concealment for those spectral lines ofthe spectrum of the audio signal, which exhibit a frequency beinggreater than a threshold frequency; and/or wherein, if error concealmentis conducted by the decoding module, the decoding module is configuredto conduct error concealment in a way that depends on whether or not aprevious bitstream payload of a previous frame preceding the currentframe encodes a signal component of the audio signal which is tonal orharmonic.

Another embodiment may have a method for decoding a current frame toreconstruct an audio signal, wherein the audio signal is encoded withinthe current frame, wherein the current frame includes a currentbitstream payload, wherein the current bitstream payload includes aplurality of payload bits, wherein the plurality of payload bits encodesa plurality of spectral lines of a spectrum of the audio signal, whereineach of the payload bits exhibits a position within the bitstreampayload, the method having the steps of: reconstructing the audiosignal, wherein, in an error concealment mode, reconstructing the audiosignal is conducted by conducting error concealment for those spectrallines of the spectrum of the audio signal, which exhibit a frequencybeing greater than a threshold frequency; and/or wherein, if errorconcealment is conducted, error concealment is conducted in a way thatdepends on whether or not a previous bitstream payload of a previousframe preceding the current frame encodes a signal component of theaudio signal which is tonal or harmonic; and outputting the audiosignal.

Another embodiment may have a non-transitory digital storage mediumhaving a computer program stored thereon to perform the method fordecoding a current frame to reconstruct an audio signal, wherein theaudio signal is encoded within the current frame, wherein the currentframe includes a current bitstream payload, wherein the currentbitstream payload includes a plurality of payload bits, wherein theplurality of payload bits encodes a plurality of spectral lines of aspectrum of the audio signal, wherein each of the payload bits exhibitsa position within the bitstream payload, the method having the steps of:reconstructing the audio signal, wherein, in an error concealment mode,reconstructing the audio signal is conducted by conducting errorconcealment for those spectral lines of the spectrum of the audiosignal, which exhibit a frequency being greater than a thresholdfrequency; and/or wherein, if error concealment is conducted, errorconcealment is conducted in a way that depends on whether or not aprevious bitstream payload of a previous frame preceding the currentframe encodes a signal component of the audio signal which is tonal orharmonic; and outputting the audio signal, when said computer program isrun by a computer.

A decoder for decoding a current frame to reconstruct an audio signal isprovided. The audio signal is encoded within the current frame. Thecurrent frame comprises a current bitstream payload. The currentbitstream payload comprises a plurality of payload bits. The pluralityof payload bits encodes a plurality of spectral lines of a spectrum ofthe audio signal. Each of the payload bits exhibits a position withinthe current bitstream payload. The decoder comprises a decoding moduleand an output interface. The decoding module is configured toreconstruct the audio signal. The output interface is configured tooutput the audio signal. The decoding module comprises an errorconcealment mode, wherein, if the decoding module is in said errorconcealment mode, the decoding module is configured to reconstruct theaudio signal by conducting error concealment for those spectral lines ofthe spectrum of the audio signal, which exhibit a frequency beinggreater than a threshold frequency. And/or, if error concealment isconducted by the decoding module, the decoding module is configured toconduct error concealment in a way that depends on whether or not aprevious bitstream payload of a previous frame preceding the currentframe encodes a signal component of the audio signal which is tonal orharmonic.

Moreover, a method for decoding a current frame to reconstruct an audiosignal is provided. The audio signal is encoded within the currentframe, wherein the current frame comprises a current bitstream payload,wherein the current bitstream payload comprises a plurality of payloadbits. The plurality of payload bits encodes a plurality of spectrallines of a spectrum of the audio signal. Each of the payload bitsexhibits a position within the current bitstream payload. The methodcomprises:

-   -   Reconstructing the audio signal, wherein, in an error        concealment mode, reconstructing the audio signal is conducted        by conducting error concealment for those spectral lines of the        spectrum of the audio signal, which exhibit a frequency being        greater than a threshold frequency; and/or, if error concealment        is conducted, error concealment is conducted in a way that        depends on whether or not a previous bitstream payload of a        previous frame preceding the current frame encodes a signal        component of the audio signal which is tonal or harmonic; and    -   Outputting the audio signal.

Furthermore, a computer program for implementing the above-describedmethod when being executed on a computer or signal processor isprovided.

In some circumstances, error concealment concepts may, e.g., be beapplied to the whole frame, e.g. if the whole frame is lost or marked asinvalid, or—even if parts of the spectrum are available—if full frameloss concealment is considered to be the best possible error concealmentstrategy.

In other circumstances, however, error concealment techniques may, e.g.,be applied to just a part of the frame, if parts of the spectrum areavailable.

Circumstances, where parts of the spectrum are available, may, forexample, occur in scalable coding, e.g. AAC scalable, AAC SLS or BSAC,where some layers are received, but others are not received(AAC=advanced audio coding, SLS=scalable to lossless, BSAC=bit slicedarithmetic coding).

Or, parts of the spectrum may, e.g., be available in redundant framecoding, where a redundant low quality copy of the lost frame isavailable, i.e. in the context of Vol P or VoLTE (see, e.g., [3] and [4]for more information on robustness and error resilience in VoIP andVoLTE; VoIP=voice over IP/voice over internet protocol; VoLTE=voice overLTE/voice over long term evolution).

Or, parts of the spectrum may, e.g., be available when selective errordetection is conducted, e.g. in AAC with RVLC (reversible variablelength coding) for the scale factor data, where certain scale factorsmight be detected to be corrupt, leading to a certain number of corruptspectra lines; or, e.g., in LC3 for DECT (digital enhanced cordlesstelecommunications), where errors in the coded representation of partsof the spectrum (representing the psychoacoustically less importantspectral range) can be separately detected.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1 illustrates a decoder for decoding a current frame to reconstructan audio signal portion of an audio signal according to an embodiment.

FIG. 2 illustrates a decoding module according to a particularembodiment.

FIG. 3 illustrates a decoding module overview according to an embodimentfor clean channel decoding.

FIG. 4 illustrates a decoding module overview according to an embodimentfor full frame loss concealment.

FIG. 5 illustrates a decoding module overview according to an embodimentfor partial frame loss concealment.

FIG. 6 illustrates a fading function according to an embodiment whichdepends on a number of lost frames in a row, and which further dependson a frame length.

FIG. 7 illustrates a threshold for sign scrambling according to anembodiment, which depends on a number of lost frames in a row and whichfurther depends on a frame length.

FIG. 8 illustrates an energy threshold factor according to anembodiment, which depends on a number of lost frames in a row and whichfurther depends on a frame length.

FIG. 9 illustrates a non-linear attenuation according to an embodiment,which depends on a number of lost frames in a row.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a decoder 100 for decoding a current frame toreconstruct an audio signal according to an embodiment.

The audio signal is encoded within the current frame. The current framecomprises a current bitstream payload. The current bitstream payloadcomprises a plurality of payload bits. The plurality of payload bitsencodes a plurality of spectral lines of a spectrum of the audio signal.Each of the payload bits exhibits a position within the currentbitstream payload.

The decoder 100 comprises a decoding module 110 and an output interface120.

The decoding module 110 is configured to reconstruct the audio signal.

The output interface 120 is configured to output the audio signal.

The decoding module 110 comprises an error concealment mode, wherein, ifthe decoding module 110 is in said error concealment mode, the decodingmodule 110 is configured to reconstruct the audio signal by conductingerror concealment for those spectral lines of the spectrum of the audiosignal, which exhibit a frequency being greater than a thresholdfrequency.

And/or, if error concealment is conducted by the decoding module 110,the decoding module 110 is configured to conduct error concealment in away that depends on whether or not a previous bitstream payload of aprevious frame preceding the current frame encodes a signal component ofthe audio signal which is tonal or harmonic.

In some embodiments, the decoding module may, e.g., be in said errorconcealment mode, if the current bitstream payload of the current framecomprises uncorrectable errors and/or if the current frame is lost. Thecurrent bitstream payload may, e.g., comprise uncorrectable errors, ifan error still exists after error correction has been conducted by thedecoder 100; or, if the current bitstream payload comprises an error andno error correction is conducted at all. A frame comprisinguncorrectable errors may, e.g., be referred to as a corrupted frame.

For example, according to an embodiment, particular error concealmentparameters may, e.g., be configured depending on whether or not saidprevious bitstream payload of said previous frame preceding the currentframe encodes said signal component of the audio signal which is tonalor harmonic.

According to an embodiment, the previous frame may, e.g., be a lastreceived frame which has been decoded by the decoding module 110 withoutconducting error concealment in the full frame loss concealment mode.

In the following, embodiments are described in more detail.

The spectrum may, e.g., be considered to be subdivided into thosespectral lines, which are available and which shall be used, and thosespectral lines, which are not available or which shall not be used (forexample, although they may, e.g., be available).

According to some embodiments, one may, e.g., proceed as follows:

In some situations, all spectral lines are available and shall be used,and thus, no frame loss concealment may, e.g., be conducted.

In other situations, certain spectral lines are available and shall beused, and partial frame loss concealment may, e.g., be conducted on themissing spectral lines.

In yet other situations, no spectral lines are available or shall beused, and full frame loss concealment may, e.g., be conducted.

In the following, error concealment depending on tonality according tosome embodiments is described.

In an embodiment, if error concealment is conducted by the decodingmodule 110, the decoding module 110 may, e.g., be configured toreconstruct a current spectrum of the audio signal by conducting errorconcealment using a plurality of signs of a previous spectrum of theaudio signal, said plurality of signs being encoded within the previousframe, wherein the decoding module 110 may, e.g., be configured toconduct error concealment in a way that depends on whether or not saidprevious frame encodes a signal component which is tonal or harmonic.For example, parameters for error concealment may, e.g., be selected ina different way depending on whether or not signal component which istonal or harmonic.

In an embodiment, said previous frame may, e.g., be a last receivedframe, which has been decoded by the decoding module 110 withoutconducting error concealment. Or, said previous frame may, e.g., be alast received frame, which has been decoded by the decoding module 110without conducting error concealment in the full frame loss concealmentmode. Or, said previous frame may, e.g., be a last received frame, whichhas been decoded by the decoding module 110 without conducting errorconcealment in the partial frame loss concealment mode or in the fullframe loss concealment mode.

According to an embodiment, if error concealment is conducted by thedecoding module 110, and if the previous bitstream payload of theprevious frame encodes a signal component which is tonal or harmonic,the decoding module 110 may, e.g., be configured to flip one or moresigns of the plurality of signs of the previous spectrum to reconstructthe current spectrum, wherein a percentage value p, indicating aprobability for a sign of the plurality of signs of the previousspectrum to be flipped by the decoding module 110 to reconstruct thecurrent spectrum, may, e.g., be between 0%≤p≤50%, wherein the decodingmodule 110 may, e.g., be configured to determine the percentage value p.In an embodiment, the decoding module 110 may, e.g., employ a sequenceof pseudo random numbers to determine whether a considered sign of theprevious spectrum shall actually be flipped or not depending on thepercentage value p.

In an embodiment, the decoding module 110 may, e.g., be configured toincrease the percentage value p depending on a number of subsequentframes. Said number of subsequent frames may, e.g., indicate for howmany subsequently (partially or fully) lost frames error concealment hasbeen conducted by the decoding module 110; or wherein said number ofsubsequent frames may, e.g., indicate for how many subsequent frameserror concealment in a particular error concealment mode has beenconducted by the decoding module 110.

In an embodiment, the decoding module 110 may, e.g., be configured todetermine the percentage value p depending on a function which dependson said number of subsequent frames, said number of subsequent framesbeing an argument of said function.

According to an embodiment, the decoding module 110 may, e.g., beconfigured to determine the percentage value p, such that p is 0%, ifsaid number of subsequent frames is smaller than a first thresholdvalue; such that 0%≤p≤50%, if said number of subsequent frames isgreater than or equal to the first threshold value and smaller than asecond threshold value, and such that p=50%, if said number ofsubsequent frames is greater than the second threshold value.

In an embodiment, the decoding module 110 may, e.g., be configured todetermine the percentage value p, such that the percentage value pincreases linearly in the range between the first threshold value andthe second threshold value depending on the number of subsequent frames.

According to an embodiment, if error concealment is conducted by thedecoding module 110, and if the previous bitstream payload of theprevious frame does not encode a signal component which is tonal orharmonic, the decoding module 110 may, e.g., be configured to flip 50%of the plurality of signs of the previous spectrum to reconstruct thecurrent spectrum.

In an embodiment, if error concealment is conducted by the decodingmodule 110, the decoding module 110 may, e.g., be configured toreconstruct a current spectrum of the audio signal by conducting errorconcealment using a plurality of amplitudes of the previous spectrum ofthe audio signal depending on whether or not the previous frame encodesa signal component which is tonal or harmonic, said plurality ofamplitudes being encoded within the previous frame.

According to an embodiment, if error concealment is conducted by thedecoding module 110, the decoding module 110 may, e.g., be configured toattenuate the plurality of amplitudes of the previous spectrum accordingto a non-linear attenuation characteristic to reconstruct the currentspectrum, wherein the non-linear attenuation characteristic depends onwhether or not the previous bitstream payload of the previous frameencodes a signal component which is tonal or harmonic. For example,parameters for the non-linear attenuation characteristic may, e.g., beselected in a different way depending on whether or not signal componentwhich is tonal or harmonic.

In an embodiment, if error concealment is conducted by the decodingmodule 110, and if the previous bitstream payload of the previous frameencodes a signal component which is tonal or harmonic, the decodingmodule 110 may, e.g., be configured to attenuate the plurality ofamplitudes of the previous spectrum depending on a stability factor,wherein said stability factor indicates a similarity between the currentspectrum and the previous spectrum; or wherein the stability factorindicates a similarity between the previous spectrum and a pre-previousspectrum of a pre-previous frame preceding the previous frame.

According to an embodiment, said pre-previous frame may, e.g., be a lastreceived frame before the previous frame, which has been decoded by thedecoding module 110 without conducting error concealment. Or, saidpre-previous frame may, e.g., be a last received frame before theprevious frame (e.g., a last but one received frame), which has beendecoded by the decoding module 110 without conducting error concealmentin the full frame loss concealment mode. Or, said pre-previous framemay, e.g., be a last received frame before the previous frame, which hasbeen decoded by the decoding module 110 without conducting errorconcealment in the partial frame loss concealment mode or in the fullframe loss concealment mode.

In an embodiment, said stability factor may, e.g., indicate saidsimilarity between the current spectrum and the previous spectrum, ifthe decoding module 110 is set to conduct partial frame lossconcealment. Said stability factor may, e.g., indicate said similaritybetween the previous spectrum and the pre-previous spectrum, if thedecoding module 110 is set to conduct full frame loss concealment.

According to an embodiment, the decoding module 110 may, e.g., beconfigured to determine an energy of a spectral bin of the previousspectrum. Moreover, the decoding module 110 may, e.g., be configured todetermine whether or not said energy of said spectral bin is smallerthan an energy threshold. If said energy is smaller than said energythreshold, the decoding module 110 may, e.g., be configured to attenuatean amplitude of the plurality of amplitudes being assigned to saidspectral bin with a first fading factor. If said energy is greater thanor equal to said energy threshold, the decoding module 110 may, e.g., beconfigured to attenuate said amplitude of the plurality of amplitudesbeing assigned to said spectral bin with a second fading factor, beingsmaller than the first fading factor. The decoding module 110 may, e.g.,be configured to conduct attenuation such that by using a smaller fadingfactor for the attenuation of one of the plurality of amplitudes, theattenuation of said one of the amplitudes is increased.

In an embodiment, the decoding module 110 may, e.g., be configured todetermine an energy of a spectral band comprising a plurality ofspectral bins of the previous spectrum. The decoding module 110 may,e.g., be configured to determine whether or not said energy of saidspectral band is smaller than an energy threshold. If said energy issmaller than said energy threshold, the decoding module 110 may, e.g.,be configured to attenuate an amplitude of the plurality of amplitudesbeing assigned to said spectral bin of said spectral band with a firstfading factor. If said energy is greater than or equal to said energythreshold, the decoding module 110 may, e.g., be configured to attenuatesaid amplitude of the plurality of amplitudes being assigned to saidspectral bin of said spectral band with a second fading factor, beingsmaller than the first fading factor. The decoding module 110 may, e.g.,be configured to conduct attenuation such that by using a smaller fadingfactor for the attenuation of one of the plurality of amplitudes, theattenuation of said one of the amplitudes is increased.

According to an embodiment, the decoding module 110 may, e.g., beconfigured to determine the first fading factor such that, depending onsaid number of subsequent frames, the first fading factor becomessmaller. Moreover, the decoding module 110 may, e.g., be configured todetermine the second fading factor such that, depending on said numberof subsequent frames, the second fading factor becomes smaller.

In an embodiment, the decoding module 110 may, e.g., be configured todetermine the first fading factor and the second fading factor, so that

  cum_fading_slow = 1, and cum_fading_fast = 1,if the current frame is a first frame among the subsequent frames, andso that if the current frame is one of the frames succeeding the firstframe among the subsequent frames, the first fading factor and thesecond fading factor may, e.g., be determined depending on said numberof subsequent frames according to:

  cum_fading_slow = cum_fading_slow * slow; cum_fading_fast =cum_fading_fast * fast;wherein cum_fading_slow on the right side of the formula is the firstfading factor of the previous frame (e.g., initialized with 1 at thefirst lost frame), wherein cum_fading_slow on the left side of theformula is the first fading factor of the current frame, whereincum_fading_fast on the right side of the formula is the second fadingfactor of the previous frame (e.g., initialized with 1 at the first lostframe), wherein cum_fading_fast on the left side of the formula is thesecond fading factor of the current frame, wherein 1>slow>fast>0.

According to an embodiment, 1>slow>fast>0.3.

In an embodiment, the decoding module 110 may, e.g., be configured todetermine said energy threshold, such that said energy threshold isequal to a first energy value, if said number of subsequent frames issmaller than a third threshold value; such that said energy threshold issmaller than said first energy value and is greater than a second energyvalue, if said number of subsequent frames is greater than or equal tothe third threshold value and smaller than a fourth threshold value; andsuch that said energy threshold is equal to said second energy value, ifsaid number of subsequent frames is greater than the fourth thresholdvalue.

According to an embodiment, the decoding module 110 may, e.g., beconfigured to determine the energy threshold, such that the energythreshold decreases linearly in the range between the third thresholdvalue and the fourth threshold value depending on the number ofsubsequent frames.

For those spectral lines, which are not available or which shall not beused, replacements are generated, whereas—depending on the tonality ofthe previously received signal, and, if this information is available,depending on the tonality of the currently received signal, a certaindegree of tonality is preserved:

More tonality is preserved, if the indicator(s) indicate, that the lastgood signal was tonal.

Less tonality is preserved, if the indicator(s) indicate, that the lastgood signal was not tonal.

Tonality is mainly represented by the relationships of the phasesbetween various bins within one frame and/or the relationships of thephases of the same bin for subsequent frames.

Some embodiments focus on the first aspect, namely that tonality ismainly represented by the relationships of the phases between variousbins within one frame.

The phases of various bins within one frame are mainly characterized bytheir signs, but also by the relationship of the amplitude of neighboredbins. Hence, the preservation of the amplitude relationship as well asthe preservation of the signs leads to a highly preserved tonality. Viceversa, the more the amplitude and/or the sign relationship is alteredbetween subsequent bins, the less tonality is preserved.

Manipulation of the signs according to some of the embodiments is nowdescribed.

From the state of the art, two approaches are known:

According to a first approach, frame repetition is applied: The signsare preserved from the previous spectrum.

In a second approach, noise substitution is conducted: The signs arescrambled relative to the previous spectrum; randomly 50% of the signsare flipped.

For unvoiced signals, noise substitution provides good results.

For voiced signals, frame repetition could be used instead, but forlonger losses, the preserved tonality (which is advantageous at thebeginning of the loss) might become annoying.

Embodiments are based on the finding that for voiced signals atransition phase between frame repetition and noise substitution isdesirable.

According to some embodiments, this may, e.g., be achieved by randomlyflipping a certain percentage of the signs per frame, where thispercentage lies between 0% and 50%, and is increasing over time.

Now, manipulation of the amplitude according to some embodiments isdescribed.

The simplest way, being frequently used in the state of the art, is theapplication of a certain attenuation factor to all frequency bins. Thisattenuation factor is increased from frame to frame to achieve a smoothfade-out. The fade-out speed might be fix or may depend on signalcharacteristics. With this approach, the relationship of the magnitudeof neighbored bins as well as the spectral shape of the whole frame ispreserved.

Also known in the state of the art is a band wise attenuation usingdifferent attenuation factors depending on the energy within each band.While this approach also preserves the relationship of the magnitude ofneighbored bins within each band, the spectral shape of the whole frameis flattened.

According to some embodiments, bins with larger values are attenuatedstronger than bins with smaller values. For this, some embodiments may,e.g., define a non-linear attenuation characteristic. This non-linearattenuation characteristic prevents overshoots, which might otherwiseoccur, since no alias cancellation is assured during the overlap-add,and alters the relationship of the magnitude of neighbored bins, whichresults in a flatter spectral shape. In order to flatten the spectralshape. Some embodiments are based on the finding that the ratio of themagnitude of neighbored bins should stay above one, if it was above onebeforehand; and that the ratio should stay below one, if it was belowone beforehand.

In order to apply this attenuation gracefully, in some embodiments, thenon-linear characteristic may, e.g., be small at the beginning of theloss and may, e.g., than subsequently be increased. In embodiments, itsadjustment over time may, e.g., depend on the tonality of the signal:According to some embodiments, for unvoiced signals, the non-linearitymay, e.g., be stronger than for voiced signals.

Such non-linear attenuation characteristic influences the spectralshape. In embodiments, the spectrum may, e.g., get flatter over time,which reduces the chance of annoying synthetic sound artefacts duringburst losses.

In the following, partial frame loss concealment according to someembodiments is described.

According to an embodiment, said error concealment mode may, e.g., be apartial frame loss concealment mode, wherein, if the decoding module 110is in the partial frame loss concealment mode, the decoding module 110may, e.g., be configured to reconstruct the audio signal withoutconducting error concealment for one or more first spectral lines of theplurality of spectral lines of the spectrum, which exhibit a frequencybeing smaller than or equal to the threshold frequency, wherein said oneor more first spectral lines have been encoded by a first group of oneor more of the plurality of payload bits. Moreover, the decoding module110 may, e.g., be configured to reconstruct the audio signal byconducting error concealment for one or more second spectral lines ofthe plurality of spectral lines of the spectrum, which exhibit afrequency being greater than the threshold frequency, wherein said oneor more second spectral lines have been encoded by a second group of oneor more of the plurality of payload bits.

In an embodiment, the decoding module 110 may, e.g., be configured todetect whether or not the current frame does not comprise any corruptedbits encoding said one or more first spectral lines of the spectrum ofthe audio signal which exhibit a frequency being smaller than or equalto the threshold frequency. Moreover, the decoding module 110 may, e.g.,be configured to detect whether or not the current frame comprises oneor more corrupted bits encoding said one or more second spectral linesof the spectrum of the audio signal which exhibit a frequency beinggreater than the threshold frequency. Said one or more corrupted bitsare one or more of the payload bits that are distorted or that arelikely to be distorted. If the current frame does not comprise anycorrupted bits encoding said one or more first spectral lines of thespectrum of the audio signal which exhibit a frequency being smallerthan or equal to the threshold frequency and if the current framecomprises said one or more corrupted bits encoding said one or moresecond spectral lines of the spectrum of the audio signal which exhibita frequency being greater than the threshold frequency, the decodingmodule 110 may, e.g., be configured to conduct error concealment in thepartial frame loss concealment mode by conducting error concealment forsaid one or more second spectral lines of the spectrum which are greaterthan the threshold frequency.

According to an embodiment, if the current frame does not comprise anycorrupted bits encoding said one or more first spectral lines of thespectrum of the audio signal which exhibit a frequency being smallerthan or equal to the threshold frequency and if the current framecomprises said one or more corrupted bits encoding said one or moresecond spectral lines of the spectrum of the audio signal which exhibita frequency being greater than the threshold frequency, the decodingmodule 110 may, e.g., be configured to reconstruct the audio signal bydecoding said first group of said one or more of the plurality ofpayload bits which encode said one or more first spectral lines of thespectrum of the audio signal which exhibit a frequency being smallerthan or equal to the threshold frequency.

In an embodiment, the decoding module 110 may, e.g., be configured todetect whether the current frame is lost, wherein, if the decoder 100has detected that the current frame is lost, the decoding module 110may, e.g., be configured to reconstruct the audio signal by conductingerror concealment for said one or more second spectral lines of thespectrum of the audio signal which exhibit a frequency being greaterthan the threshold frequency. Moreover, the decoding module 110 may,e.g., be configured to decode without conducting error concealment forsaid first group, said first group of said one or more of the pluralityof payload bits which encode said one or more first spectral lines forsaid one or more first frequencies of the spectrum of the audio signalbeing smaller than or equal to the threshold frequency, wherein saidfirst group of said one or more of the plurality of payload bits are oneor more payload bits of a redundant frame being different from thecurrent frame.

In an embodiment, the redundant frame may, e.g., be a bandwidth limitedversion of the current frame. For example, the redundant frame may,e.g., provide data (e.g., a reduced data set compared to the currentframe) that encodes the audio signal for a same time period as thecurrent frame. This data may, e.g., be different for the plurality ofpayload bits which encodes the audio signal of said one or more firstspectral lines for said one or more first frequencies of the spectrum ofthe audio signal being smaller than or equal to the threshold frequencyas they are encoded with less bits than the current frame of said firstfrequencies of the spectrum for the same time period of the currentframe.

In an embodiment, if the decoding module 110 is configured to conducterror concealment in a full frame loss concealment mode, the decodingmodule 110 is configured to conduct error concealment for all spectrallines of the (whole) spectrum (as otherwise reconstructable by allpayload bits of the current bitstream payload of the current frame).

According to an embodiment, the plurality of payload bits is a pluralityof current payload bits. If the decoding module 110 is in the partialframe loss concealment mode, the decoding module 110 may, e.g., beconfigured to conduct error concealment for said one or more secondspectral lines of the spectrum of the audio signal which exhibit afrequency being greater than the threshold frequency, using one or morestored spectral lines which have been encoded by one or more previouspayload bits of the previous bitstream payload of the previous frame.

In an embodiment, the spectrum may, e.g., be a current quantizedspectrum. If the decoding module 110 is conducting error concealment inthe partial frame loss concealment mode, the decoding module 110 may,e.g., be configured to conduct error concealment for said one or moresecond spectral lines of the spectrum of the audio signal which exhibita frequency being greater than the threshold frequency, to obtain one ormore intermediate spectral lines of said current quantized spectrum.

According to an embodiment, the spectrum is a current quantizedspectrum. If the decoding module 110 is conducting error concealment inthe partial frame loss concealment mode, the decoding module 110 may,e.g., be configured to conduct error concealment for said one or moresecond spectral lines of the spectrum of the audio signal which exhibita frequency being greater than the threshold frequency, to obtain one ormore intermediate spectral lines of said current quantized spectrum,wherein the decoding module 110 may, e.g., be configured to rescale theone or more intermediate spectral lines using a rescaling factor toreconstruct the audio signal.

In an embodiment, the decoding module 110 may, e.g., be configured todetermine the rescaling factor depending on at least one of

-   -   a global gain being encoded within said current bitstream        payload and    -   a global gain being encoded within said previous bitstream        payload, and    -   an energy of a previous quantized spectrum of said previous        frame, an energy of a previous decoded spectrum of said previous        frame, and    -   an energy of said current quantized spectrum of said current        frame.

According to an embodiment, the decoding module 110 may, e.g., beconfigured to determine the rescaling factor depending on whether or not

-   -   a mean energy of spectral bins of the previous decoded spectrum        of the previous frame starting from a first spectral bin that        cannot be reconstructed without conducting error concealment up        to a top of the spectrum is greater than or equal to a mean        energy of spectral bins of the previous decoded spectrum of the        previous frame starting from zero up to said last spectral bin        that can be reconstructed without conducting error concealment,        or    -   an energy of spectral bins of said current quantized spectrum of        the current frame starting from zero up to said last spectral        bin that can be reconstructed without conducting error        concealment is greater than or equal to an energy of spectral        bins of the previous quantized spectrum of the previous frame        starting from zero up to said last spectral bin that can be        reconstructed without conducting error concealment.

In an embodiment,

-   -   if the mean energy of the spectral bins of the previous decoded        spectrum of the previous frame starting from said first spectral        bin that cannot be reconstructed without conducting error        concealment up to a top of the spectrum is smaller than the mean        energy of the spectral bins of the previous decoded spectrum of        the previous frame starting from zero up to said last spectral        bin that can be reconstructed without conducting error        concealment, and    -   if the energy of spectral bins of said current quantized        spectrum of the current frame starting from zero up to said last        spectral bin that can be reconstructed without conducting error        concealment is smaller than the energy of the spectral bins of        the previous quantized spectrum of the previous frame starting        from zero up to said last spectral bin that can be reconstructed        without conducting error concealment,    -   the decoding module 110 may, e.g., be configured to determine        the rescaling factor such that the rescaling factor is equal to        the square root of the ratio of    -   the energy of the spectral bins of the current quantized        spectrum starting from zero up to said last spectral bin that        can be reconstructed without conducting error concealment        multiplied with the square of a gain factor of the current        frame,        -   to    -   the energy of the spectral bins of the previous quantized        spectrum starting from zero up said last spectral bin that can        be reconstructed without conducting error concealment multiplied        with the square of a gain factor of the previous frame.

According to an embodiment, the decoding module 110 may, e.g., beconfigured to determine the rescaling factor, being a total rescalingfactor, depends on a global gain rescaling factor, wherein the decodingmodule 110 may, e.g., be configured to determine the global gainrescaling factor according to

${fac_{gg}} = {\frac{gg_{prev}}{gg}.}$

wherein gg indicates a global gain of the current frame, and whereingg_(prev) indicates a global gain of said previous frame, and whereinfac_(gg) is the global gain rescaling factor.

In an embodiment,

${{{if}\mspace{14mu}\frac{1}{k_{be}}{\sum_{k = 0}^{k_{be} - 1}{{\overset{\hat{}}{X}}_{prev}(k)}^{2}}} \leq {\frac{1}{N_{F} - k_{be}}{\sum_{k = k_{be}}^{N_{F} - 1}{{\overset{\hat{}}{X}}_{prev}(k)}^{2}}}},{or}$${{{if}\mspace{14mu}{{gg}_{prev}^{2} \cdot {\sum_{k = 0}^{k_{be} - 1}{\;(k)^{2}}}}} \leq {g{g^{2} \cdot {\sum_{k = 0}^{k_{be} - 1}{\;(k)^{2}}}}}},$

the decoding module 110 may, e.g., be configured to determine that thetotal rescaling factor is equal to the global gain rescaling factor,wherein k indicates a spectral bin, wherein k_(be) indicates a firstspectral bin that could not be recovered, wherein N_(F) indicates anumber of spectral lines, wherein

(k) indicates the previous quantized spectrum of the previous framebeing a last non-full frame loss concealment frame, wherein

(k) indicates the current quantized spectrum of the current frame,wherein {circumflex over (X)}_(prev)(k) indicates the previous decodedspectrum of the previous frame being said last non-full frame lossconcealment frame.

According to an embodiment,

$\begin{matrix}{if} & {{{\frac{1}{k_{be}}{\sum\limits_{k = 0}^{k_{be} - 1}{{\hat{X}}_{prev}(k)}^{2}}} > {\frac{1}{N_{F} - k_{be}}{\sum\limits_{k = k_{be}}^{N_{F} - 1}{{\hat{X}}_{prev}(k)}^{2}}}},{and}} \\{if} & {{{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}} > {{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}},}\end{matrix}$

the decoding module 110 may, e.g., be configured to determine that thetotal rescaling factor moreover depends on an energy rescaling factor

${{fac}_{ener} = \sqrt{\frac{{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}}},$

which may, e.g., be employed to form the total rescaling factor

${fac} = {{{fac}_{gg} \cdot {fac}_{ener}} = {\sqrt{\frac{\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}{\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}.}}$

wherein fac_(ener) indicates the energy rescaling factor, wherein kindicates a spectral bin, wherein k_(be) indicates a first spectral binthat could not be recovered, wherein N_(F) indicates a number ofspectral lines, wherein

(k) indicates the previous quantized spectrum of the previous framebeing a last non-full frame loss concealment frame, wherein

(k) indicates the current quantized spectrum of the current frame,wherein {circumflex over (X)}_(prev)(k) indicates the previous decodedspectrum of the previous frame being said last non-full frame lossconcealment frame.

In a scenario, where partial frame loss concealment is applied, it isassumed or it has been determined that the more sensitive bits of thebitstream payload were error-free.

In embodiments, the quantized spectrum

(k) of the current frame may, e.g., be restored up to a certainfrequency bin, here referred to as frequency bin k_(be)−1. Partial frameloss concealment thus just conceals the quantized spectral lines abovethis frequency.

When conducting partial frame loss concealment according to someembodiments, the spectral lines of the quantized spectrum of the lastnon-FFLC frame

(k) may, e.g., be reused (FFLC=full frame loss concealment).

To prevent high-energy artefacts in transition frames changing inenergy, the concealed spectral lines are subsequently rescaled, whereasthe resulting rescaling factor may, e.g., depend on at least one of

-   -   a) global gains;    -   b) energies of the spectra;

Advantageously, the resulting rescaling factor may, e.g., depend on bothglobal gains and energies of the spectra.

The rescaling factor based on the global gains equals the ratio of theprevious global gain to the current global gain.

The rescaling factor based on the energies is initialized with 1 (e.g.,no rescaling is conducted/e.g., rescaling has no effect):

-   -   If the mean energy of the spectral bins of the previous decoded        spectrum starting from the frequency bin k_(be) (a first        spectral bin that cannot be reconstructed without conducting        error concealment) up to the top of the spectrum is larger or        equal than the mean energy of the spectral bins of the previous        decoded spectrum starting from zero up to the frequency bin        k_(be)−1 (a last spectral bin that can be reconstructed without        conducting error concealment); or    -   If the energy of the spectral bins of the current quantized        spectrum starting from zero up to the frequency bin k_(be)−1 is        larger than or equal to the energy of the spectral bins of the        previous quantized spectrum starting from zero up to the        frequency bin k_(be)−1.

Otherwise, the rescaling factor equals the square root of the ratio of

-   -   The energy of the spectral bins of the current quantized        spectrum starting from zero up to the frequency bin k_(be)−1        multiplied with the square of the gain factor of the current        frame; to    -   the energy of the spectral bins of the previous quantized        spectrum starting from zero up to the frequency bin k_(be)−1        multiplied with the square of the gain factor of the previous        frame.

When multiplying both factors in this case, the gain factors cancel eachother out. Thus, the rescaling factor subsequently equals the squareroot of the ratio of

-   -   The energy of the spectral bins of the current quantized        spectrum starting from zero up to the frequency bin k_(be)−1; to    -   the energy of the spectral bins of the previous quantized        spectrum starting from zero up to the frequency bin k_(be)−1.

After that, the concealed quantized spectrum may, e.g., be handled as anerror-free quantized spectrum. This means the subsequent decoderoperations like noise filling, noise shaping or any other operationswhose parameters are stored in the error-free bitstream payload may,e.g., be applied afterwards. Thus, possible concealing artefacts aremitigated.

Subsequently, a similar fading process as described above may, forexample, be applied on the spectrum starting from the frequency bink_(be) up to the top of the spectrum, for example, a possibly availabletonal character may, e.g., be faded towards noise; and/or, for example,a possibly pronounced spectral shape may, e.g., be flattened; and/or theenergy may, e.g., be decreased.

In the following, embodiments are described in detail.

FIG. 2 illustrates a decoding module 110 according to particularembodiments.

The decoding module 110 of FIG. 2 comprises a decoded spectrum storagemodule 330, and, optionally, a quantized spectrum storage module 310, apartial frame repetition & rescaling module 320 and a fade-out and signscrambling module 340. Particular details of these (sub)modules of thespecific decoding module 110 of FIG. 2 are described with reference toFIG. 3-FIG. 5.

FIG. 3-FIG. 5 provide high-level overview of the LC3 decoder(exemplarily used as state-of-the-art transform coder that is modifiedin inventive ways) according to embodiments. In particular, FIG. 3-FIG.5 provide different kinds of specific embodiments for a decoding module110.

In an embodiment, the decoding module 110 may, e.g., comprise aquantized spectrum storage module 310 configured for storing a quantizedspectrum of the audio signal, wherein the quantized spectrum storagemodule 310 is configured to provide a last non-full frame lossconcealment quantized spectrum. Moreover, the decoding module 110 may,e.g., comprise a decoded spectrum storage module 330 configured forstoring a decoded spectrum of the audio signal, wherein the decodedspectrum storage module 330 is configured to provide a last non-fullframe loss concealment decoded spectrum.

FIG. 3 illustrates a decoding module 110 overview according to anembodiment for clean channel decoding. In particular, FIG. 3 shows thenormal decoder operation. The processing blocks that may be used for thefull frame loss concealment as well as for the partial frame lossconcealment are processing blocks 310 and 330.

The quantized spectrum storage module 310 may, e.g., be configured forthe storage of the quantized spectrum: The quantized spectrum storagemodule 310 stores the last non-FFLC quantized spectrum to allow itsre-usage in the case of partial frame loss concealment.

The decoded spectrum storage module 330 is configured for the storage ofthe spectrum (e.g., referred to as the decoded spectrum): Thisprocessing block stores the last non-FFLC spectrum to allow its re-usagein the case of full frame loss concealment. It may, e.g., also be usedfor the rescaling during the partial frame loss concealment.

In an embodiment, the decoding module 110 may, e.g., comprise a decodedspectrum storage module 330 configured for storing a decoded spectrum ofthe audio signal, wherein the decoded spectrum storage module 330 isconfigured to provide a last non-full frame loss concealment decodedspectrum. Moreover, the decoding module 110 may, e.g., comprise afade-out and sign scrambling module 340 being configured for fade-outand sign scrambling on spectral lines of the spectrum.

In addition, according to an embodiment, the decoding module 110 may,e.g., comprise a quantized spectrum storage module 310 configured forstoring a quantized spectrum of the audio signal, wherein the quantizedspectrum storage module 310 is configured to provide a last non-fullframe loss concealment quantized spectrum. Furthermore, the decodingmodule 110 may, e.g., be comprise a partial frame repetition & rescalingmodule 320 configured for partial frame repetition and rescaling,wherein the partial frame repetition & rescaling module 320 isconfigured to complement the spectrum by adding spectral lines, whichcould not be decoded by the decoding module 110, wherein the partialframe repetition & rescaling module 320 is configured to re-scale saidspectral lines.

FIG. 4 illustrates a decoding module 110 overview according to anembodiment for full frame loss concealment. In particular, FIG. 4depicts an embodiment configured for conducting full frame lossconcealment. The processing blocks that may be used for the full frameloss concealment are processing blocks 330 and 340. Processing blocks330 and 340 may, e.g., have the following tasks.

The decoded spectrum storage module 330 may, e.g., be configured for thestorage of the spectrum (e.g., again referred to as the decodedspectrum): This processing block 330 provides the last non-FFLCspectrum.

The fade-out and sign scrambling module 340 may, e.g., be configured forfade-out and sign scrambling: This processing block is configured tocreate the spectrum by processing the spectral lines of the lastnon-FFLC frame, as described below.

FIG. 5 illustrates a decoding module 110 overview according to anembodiment for partial frame loss concealment.

In particular, FIG. 5 shows the application of partial frame lossconcealment. The processing blocks that may be used for the partialframe loss concealment are processing blocks 310, 320, 330 and 340.These processing blocks 310, 320, 330 and 340 have the following tasks:

The quantized spectrum storage module 310 may, e.g., be configured forthe storage of the quantized spectrum: The quantized spectrum storagemodule 310 may, e.g., be configured to provide the last non-FFLCquantized spectrum.

The partial frame repetition & rescaling module 320 may, e.g., beconfigured for partial frame repetition and rescaling: This processingblock may, e.g., be configured to complement the spectrum by addingthose spectral lines, which could not be decoded. Afterwards, thosespectral lines may, e.g., be re-scaled and values below a certainthreshold are quantized to zero, as explained below.

The decoded spectrum storage module 330 may, e.g., be configured for thestorage of the spectrum (e.g., again referred to as the decodedspectrum): The decoded spectrum storage module 330 may, e.g., beconfigured to provide the last non-FFLC spectrum, which may, e.g., beused for computing the rescaling factor.

The fade-out and sign scrambling module 340 may, e.g., be configured forfade-out and sign scrambling: The fade-out and sign scrambling module340 may, e.g., be configured to process the spectral lines, which werepreviously provided by the partial frame loss concealment. It isexplained below.

In the following, error concealment depending on tonality according tosome embodiments is described in more detail.

At first, a fading function according to some embodiments is provided.

For the fading processes implemented for the sign scrambling and thenon-linear attenuation as described below, a function depending on thenumber of subsequently lost frames (nbLostFrameslnRow) may, e.g., beemployed, that is one (1) up to a certain value (plc_start_inFrames),that is zero (0) from a certain value (plc_end_inFrames); and thatdecreases linearly between one and zero (1>x>0) betweenplc_start_inFrames and plc_end_inFrames.

A particular embodiment may, e.g., be implemented as follows:

  plc_duration_inFrames = plc_end_inFrames − plc_start_inFrames; x =max(plc_start_inFrames, (min (nbLostFramesInRow, plc_end_inFrames))); m= −1 / plc_duration_inFrames; b = − plc_end_inFrames; linFuncStartStop =m * (x + b);where:

-   plc_start_inFrames—number of subsequently lost frames, up to which    the value of linFuncStartStop equals 1-   plc_end_inFrames—number of subsequently lost frames, from which the    value of linFuncStartStop equals 0-   linFuncStartStop—value of fading function

The start value and the end value might be chosen differently dependingon the signal characteristic (e.g. voiced vs. unvoiced) and depending onthe frame loss concealment (e.g. PFLC vs. FFLC) (PFLC=partial frame lossconcealment; FFLC=full frame loss concealment).

FIG. 6 illustrates a fading function according to an embodiment whichdepends on a number of lost frames in a row (a number of subsequentlylost frames).

In particular, FIG. 6 provides an example of this fading function, whichis configured to decrease linearly between 20 ms and 60 ms.

In the following, manipulation of signs according to some embodiments isdescribed in more detail.

As a prerequisite, a threshold for sign scrambling may, e.g., bedetermined based on the fading value (linFuncStartStop) as derivedabove.

randThreshold=−32768*linFuncStartStop;

FIG. 7 illustrates a threshold for sign scrambling according to anembodiment, which depends on a number of lost frames in a row (a numberof subsequently lost frames) and which further depends on a framelength.

In particular, FIG. 7 provides an example for a threshold which dependson a number of consecutively lost frames using the fading function,where a threshold of 0 corresponds to 50% sign flipping, whereas athreshold of −32768 corresponds to 0% sign flipping.

An embodiment may, e.g., be realized by the following pseudo code:

  for k=k_(be).. N_(F) − 1  seed = 16831+seed*12821;  seed =seed−round(seed*2{circumflex over ( )}−16)*2{circumflex over ( )}16;  ifseed==32768   seed=−32768;  end  if (seed < 0 && pitch_present == 0) ||seed < randThreshold   spec(k) = −spec_prev(k);  else   spec(k) =spec_prev(k);  end endwhere:

-   k—spectral bin-   k_(be)—first spectral bin which could not be recovered-   N_(F)—number of spectral lines-   seed—random value with the exemplary initial value of 24607-   pitch_present—information, whether the signal in the current frame    is tonal-   spec_prev(k)—spectral value of bin k in last good frame (also    referred to as {circumflex over (X)}_(prev)(k))-   spec(k)—spectral value of bin k in current frame.

In this example, the seed (i.e. the random value) varies between 32768and −32768. For unvoiced signals (pitch_present==0), the threshold forthe sign inversion is zero, which leads to a 50% probability. For voicedsignals, a variable threshold (randThreshold) is applied, which liesbetween −32768 (0% probability of sign inversion) and zero (50%probability of sign inversion).

In the following, a manipulation of the amplitude according to someembodiments is described in more detail.

In a particular embodiment, two attenuation factors may, e.g., bedefined depending on a stability measure, for example, as follows:

  slow = 0.8 + 0.2 * stabFac; fast = 0.3 + 0.2 * stabFac;wherein stabFac indicates a stability value between the last and secondlast frame in FFLC case or current and last frame in PFLC case.

The stability factor may, for example, represent a similarity betweentwo signals, for example, between the current signal and a past signal.For example, the stability factor may, e.g., be bounded by [0:1]. Astability factor close to 1 or 1 may, e.g., mean that both signals arevery similar and a stability factor close to 0 or 0 may, e.g., mean thatboth signals are very different. The similarity may, for example, becalculated on the spectral envelopes of two audio signals.

The stability factor θ may, for example, be calculated as:

$\theta = {1.25 - {\frac{1}{25}{\sum\limits_{k = 0}^{N - 1}( {{{scfQ}_{curr}(k)} - {{scfQ}_{prev}(k)}} )^{2}}}}$

wherein:

-   scfQcurr indicates a scalefactor vector of the current frame, and-   scfQ_(prev) indicates a scalefactor vector of the previous frame-   N indicates the number of scalefactors within the scalefactor    vectors-   θ indicates a stability factor, which is bounded by 0≤θ≤1-   k indicates an index for a scalefactor vector

In some embodiments, stabFac may, for example, be used differently forFFLC and for PFLC; i.e. it could be set a value between 0 and 1depending on the stability for FFLC, whereas it could be set to 1 forPFLC.

Subsequently, corresponding cumulative attenuation factors(cum_fading_slow and cum_fading_fast, initialized with 1 at thebeginning of each burst loss) may, for example, be derived, which may,e.g., change from frame to frame, for example, as follows:

  cum_fading_slow = cum_fading_slow * slow; cum_fading_fast =cum_fading_fast * fast;wherein: cum_fading_slow indicates a slow cumulative damping factor; andwherein cum_fading_fast indicates a fast cumulative damping factor.

In an embodiment, the accumulation may, e.g., be done just for FFLC, butnot for PFLC.

Furthermore, according to an embodiment, values for a first threshold(ad_ThreshFac_start) and a last threshold (ad_ThreshFac_end) may, e.g.,be defined. In some embodiments, these values may, e.g., be chosenheuristically. Usually, both values may, e.g., be larger than one (1),and the first threshold is larger than the last threshold. Based onthose two threshold limits, the threshold for the current frame(ad_threshFac) may, e.g., be determined based on the fading value(linFuncStartStop) as derived above:

  ad_ThreshFac_start = 10; ad_ThreshFac_end = 1.2; ad_threshFac =(ad_ThreshFac_start − ad_ThreshFac_end) * linFuncStartStop +ad_ThreshFac_end;wherein ad_ThreshFac_start indicates a first factor to mean energy,above which a stronger attenuation is applied; and whereinad_ThreshFac_stop indicates a last factor to mean energy, above which astronger attenuation is applied.

The threshold adjustment could be done just for FFLC, but not for PFLC.In this case, the threshold would be fix for subsequent frames.

FIG. 8 illustrates an energy threshold factor according to anembodiment, which depends on a number of lost frames in a row and whichfurther depends on a frame length.

In particular, FIG. 8 provides an example for a threshold factordepending on the number of consecutively lost frames using the fadingfunction, wherein the threshold factor decreases from 10 to 1.2 between20 ms and 60 ms.

In a particular embodiment, the adaptive fading is operating on a bingranularity. It could be realized as follows:

frame_energy = mean(spec(k_(be).. N_(F) − 1).{circumflex over ( )}2);energThreshold = ad_threshFac * frame_energy; for k=k_(be).. N_(F) − 1 if (spec(k){circumflex over ( )}2) < energThreshold   m =cum_fading_slow;   n = 0;  else   m = cum_fading_fast;   n =(cum_fading_slow−cum_fading_fast) * sqrt(energThreshold) *  sign(spec_prev(k));  end  spec(k) = m * spec_prev(k) + n; endwhere:

-   k—spectral bin-   k_(be)—first spectral bin which could not be recovered-   N_(F)—number of spectral lines-   spec_prev(k)—spectral value of bin k in last good frame (below    referred to as {circumflex over (X)}_(prev)(k))-   spec(k)—spectral value of bin k in current frame.

The derivation of n in the else path makes sure, that the attenuationcurve keeps larger values larger, and smaller values smaller.

FIG. 9 depicts an example where cumulative damping is applied. Possibleinput values in the example are between 0 and 1000. n=0 refers to areceived frame and provides some sort of reference. In the example, theinitial slow attenuation factor is set to 0.9, and the initial fastattenuation factor is set to 0.4 (stabFac=0.5). In the second frame, thesquares of those values are used, and so on, which makes subsequentcurves flatter. At the same time, the threshold may, e.g., be reduced,which moves the twist of the successive curves further to the left.

In another particular embodiment, the adaptive fading is operating bandwise. In that example, band wise energies are derived, and an adaptivedamping is just applied to bins in bands, which are above the mean overall bands. In those cases, the energy of that band may, e.g., be used asthreshold for bin wise adaptive damping. An exemplary implementationmay, for example, be realized as follows:

  bin_energy_per_band = zeros(ceil((N_(F) − k_(be) )/8),1); idx = 1; fork=k_(be):8:(N_(F)−7)  bin_energy_per_band(idx) =mean(spec(k:k+7).{circumflex over ( )}2);  idx = idx + 1; endenergThreshold = ad_threshFac * mean(bin_energy_per_band); idx = 1; fork=k_(be):8:(N_(F)−7)  if bin_energy_per_band(idx) < energThreshold   m =cum_fading_slow;   spec(k:k+7) = m * spec_prev(k:k+7);  else   forj=k:k+7    if (spec(j){circumflex over ( )}2) < bin_energy_per_band(idx)    m = cum_fading_slow;     n = 0;    else     m = cum_fading_fast;    n = (cum_fading_slow−cum_fading_fast) *     sqrt(energThreshold) *sign(spec_prev(j));    end    spec(j) = m * spec_prev(j) + n;   end  end idx = idx + 1; endwherein:

-   k, j—spectral bin-   k_(be)—first spectral bin which could not be recovered-   N_(F)—number of spectral lines-   spec_prev(k)—spectral value of bin k in last good frame (below    referred to as {circumflex over (X)}_(prev)(k))-   spec(k)—spectral value of bin k in current frame-   idx—band index-   bin_energy_per_band—bin energy per band

The storage of the spectral lines during a received frame as well as theinsertion of the spectral lines during a (partially or fully) lost framemay, in general, be performed at any place between the decoding of thespectrum based on the information provided in the bitstream and thetransformation back into the time domain. Referring to LC3, it mayespecially be performed, e.g., before or after SNS decoding(SNS=Spectral Noise Shaping), e.g., before or after TNS decoding(TNS=Temporal Noise Shaping), e.g., before or after the application ofthe global gain, and/or, e.g., before or after the noise filling.

The selection of the advantageous location may also vary depending onthe availability of additional information for the partially or fullylost frame. It may, e.g., be performed at the beginning of the signalprocessing in the case of a partially lost frame (partial frame lossconcealment); since in this case the parameters for the subsequentsignal processing steps are available. It may, e.g., be performed at alater stage in the case of a fully lost frame (full frame lossconcealment), since in this case no parameters of the subsequent signalprocessing steps are available. It may, however, e.g., still beperformed before the SNS decoding, since this step allows a dedicatedspectral shaping.

In the following, partial frame loss concealment according to someembodiments is described in more detail.

A particular implementation for partial frame loss concealment may,e.g., first apply the rescaling factor and then may, e.g., quantizespectral bins below a certain threshold to zero. This is shown in thefollowing example pseudo code:

  for k=k_(be).. N_(F) − 1  

 (k) = 

 (k) · fac;  if |

 (k) | < threshold    

 (k) = 0;wherein:

-   k—spectral bin-   N_(F)—number of spectral lines-   k_(be)—first spectral bin which could not be recovered-   (k)—quantized spectral line k of the current frame-   (k)—quantized spectral line k of the last non-FFLC frame-   fac—rescaling factor-   threshold—threshold value with the exemplary value of 0.625 to    quantize to zero.

The rescaling factor depending on the global gains fac_(gg) is derivedas the ratio between current and the past global gain:

${fac}_{gg} = {\frac{{gg}_{prev}}{gg}.}$

The rescaling factor depending on the energies fac_(ener) is initializedwith 1. If the following conditions are met

$\begin{matrix}{{\frac{1}{k_{be}}{\sum\limits_{k = 0}^{k_{be} - 1}{{\hat{X}}_{prev}(k)}^{2}}} > {\frac{1}{N_{F} - k_{be}}{\sum\limits_{k = k_{be}}^{N_{F} - 1}{{{\hat{X}}_{prev}(k)}^{2}.}}}} & 1 \\{{{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}} > {{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}},} & 2\end{matrix}$

this rescaling factor is set to the root of the ratio between thecurrent and the past quantized spectrum multiplied with the square oftheir corresponding global gains:

${fac}_{ener} = {\sqrt{\frac{{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}}.}$

The total rescaling factor is derived as

fac=fac _(gg) ·fac _(ener).

When fac_(ener)≠1, this leads to (the global gain values cancel eachother out):

${fac} = {{\frac{{gg}_{prev}}{gg} \cdot \sqrt{\frac{{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}}} = {\sqrt{\frac{\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}{\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}.}}$

The variables in the above equations have the following meaning:

-   k—spectral bin-   k_(be)—first spectral bin that could not be recovered-   N_(F)—number of spectral lines-   (k) quantized spectrum of last non-FFLC frame-   (k)—quantized spectrum of the current frame-   {circumflex over (X)}prev(k)—decoded spectrum of the last non-FFLC    frame-   gg—global gain of the current frame (if the quantized spectrum coded    in the bitstream is rescaled with a global gain)-   gg_(prev)—global gain of the last non-FFLC frame (if the quantized    spectrum coded in the bitstream is rescaled with a global gain).

The following example pseudo code shows the determination of therescaling factor according to an exemplary implementation:

  fac = gg_(prev)/gg; mean_nrg_high = mean({circumflex over(X)}_(prev)(k_(be) : N_(F) − 1).{circumflex over ( )}2); mean_nrg_low =mean({circumflex over (X)}_(prev)(0 : k_(be)−1).{circumflex over ( )}2);if (mean_nrg_low > mean_nrg_high)  ener_prev = sum(

 (0 : k_(be)−1).{circumflex over ( )}2);  ener_curr = sum(

 (0 : k_(be)−1).{circumflex over ( )}2);  ifener_prev*gg_(prev){circumflex over ( )}2 > ener_curr*gg{circumflex over( )}2   fac = sqrt(ener_curr/ener_prev);wherein:

-   fac—rescaling factor-   gg—global gain of the current frame (if the quantized spectrum coded    in the bitstream is rescaled with a global gain)-   gg_(prev)—global gain of the last non-FFLC frame (if the quantized    spectrum coded in the bitstream is rescaled with a global gain)-   k_(be)—first spectral bin which could not be recovered-   N_(F)—number of spectral lines-   {circumflex over (X)}_(prev)—decoded spectrum of the last non-FFLC    frame-   —quantized spectrum of the last non-FFLC frame-   (k)—quantized spectrum of the current frame-   sqrt—square root function.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, one or more ofthe most important method steps may be executed by such an apparatus.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software or at leastpartially in hardware or at least partially in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitory.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods may be performed by any hardware apparatus.

The apparatus described herein may be implemented using a hardwareapparatus, or using a computer, or using a combination of a hardwareapparatus and a computer.

The methods described herein may be performed using a hardwareapparatus, or using a computer, or using a combination of a hardwareapparatus and a computer.

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which fall withinthe scope of this invention. It should also be noted that there are manyalternative ways of implementing the methods and compositions of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutationsand equivalents as fall within the true spirit and scope of the presentinvention.

REFERENCES

-   [1] P. Lauber and R. Sperschneider, “Error Concealment for    Compressed Digital Audio,” in Audio Engineering Society, 2001.-   [2] J. Lecomte and A. Tomasek, “ERROR CONCEALMENT UNIT, AUDIO    DECODER, AND RELATED METHOD AND COMPUTER PROGRAM FADING OUT A    CONCEALED AUDIO FRAME OUT ACCORDING TO DIFFERENT DAMPING FACTORS FOR    DIFFERENT FREQUENCY BANDS”, WO 2017/153299 A2, published 2017.-   [3] A. Rämö, A. Kurittu and H. Toukomaa, “EVS Channel Aware Mode    Robustness to Frame Erasures,” in Interspeech 2016, San Francisco,    Calif., USA, 2016.-   [4] A. Venkatraman, D. J. Sinder, S. Shaminda, R. Vivek, D.    Duminda, C. Venkata, V. Imre, K. Venkatesh, S. Benjamin, L.    Jeremie, Z. Xingtao and M. Lei, “Improved Error Resilience for VoLTE    and VoIP with 3GPP EVS Channel Aware Coding,” in ICASSP 2015.-   [5] M. Schnabel, G. Markovic, R. Sperschneider, C. Helmrich and J.    Lecomte, “Apparatus and method realizing a fading of an mdct    spectrum to white noise prior to fdns application”. European Patent    EP 3011559 B1, published 2017.

1. A decoder for decoding a current frame to reconstruct an audiosignal, wherein the audio signal is encoded within the current frame,wherein the current frame comprises a current bitstream payload, whereinthe current bitstream payload comprises a plurality of payload bits,wherein the plurality of payload bits encodes a plurality of spectrallines of a spectrum of the audio signal, wherein each of the payloadbits exhibits a position within the current bitstream payload, whereinthe decoder comprises a decoding module configured to reconstruct theaudio signal, and an output interface configured to output the audiosignal, wherein the decoding module comprises an error concealment mode,wherein, if the decoding module is in said error concealment mode, thedecoding module is configured to reconstruct the audio signal byconducting error concealment for those spectral lines of the spectrum ofthe audio signal, which exhibit a frequency being greater than athreshold frequency; and/or wherein, if error concealment is conductedby the decoding module, the decoding module is configured to conducterror concealment in a way that depends on whether or not a previousbitstream payload of a previous frame preceding the current frameencodes a signal component of the audio signal which is tonal orharmonic.
 2. A decoder according to claim 1, wherein said errorconcealment mode is a partial frame loss concealment mode, wherein, ifthe decoding module is in the partial frame loss concealment mode, thedecoding module is configured to reconstruct the audio signal withoutconducting error concealment for one or more first spectral lines of theplurality of spectral lines of the spectrum, which exhibit a frequencybeing smaller than or equal to the threshold frequency, wherein said oneor more first spectral lines have been encoded by a first group of oneor more of the plurality of payload bits, and to reconstruct the audiosignal by conducting error concealment for one or more second spectrallines of the plurality of spectral lines of the spectrum, which exhibita frequency being greater than the threshold frequency, wherein said oneor more second spectral lines have been encoded by a second group of oneor more of the plurality of payload bits.
 3. A decoder according toclaim 2, wherein the decoding module is configured to detect whether ornot the current frame does not comprise any corrupted bits encoding saidone or more first spectral lines of the spectrum of the audio signalwhich exhibit a frequency being smaller than or equal to the thresholdfrequency, wherein the decoding module is configured to detect whetheror not the current frame comprises one or more corrupted bits encodingsaid one or more second spectral lines of the spectrum of the audiosignal which exhibit a frequency being greater than the thresholdfrequency, wherein said one or more corrupted bits are one or more ofthe payload bits that are distorted or that are likely to be distorted,and wherein, if the current frame does not comprise any corrupted bitsencoding said one or more first spectral lines of the spectrum of theaudio signal which exhibit a frequency being smaller than or equal tothe threshold frequency and if the current frame comprises said one ormore corrupted bits encoding said one or more second spectral lines ofthe spectrum of the audio signal which exhibit a frequency being greaterthan the threshold frequency, the decoding module is configured toconduct error concealment in the partial frame loss concealment mode byconducting error concealment for said one or more second spectral linesof the spectrum which are greater than the threshold frequency.
 4. Adecoder according to claim 3, wherein, if the current frame does notcomprise any corrupted bits encoding said one or more first spectrallines of the spectrum of the audio signal which exhibit a frequencybeing smaller than or equal to the threshold frequency and if thecurrent frame comprises said one or more corrupted bits encoding saidone or more second spectral lines of the spectrum of the audio signalwhich exhibit a frequency being greater than the threshold frequency,the decoding module is configured to reconstruct the audio signal bydecoding said first group of said one or more of the plurality ofpayload bits which encode said one or more first spectral lines of thespectrum of the audio signal which exhibit a frequency being smallerthan or equal to the threshold frequency.
 5. A decoder according toclaim 2, wherein the decoding module is configured to detect whether thecurrent frame is lost, wherein, if the decoder has detected that thecurrent frame is lost, the decoding module is configured to reconstructthe audio signal by conducting error concealment for said one or moresecond spectral lines of the spectrum of the audio signal which exhibita frequency being greater than the threshold frequency, and by decodingwithout conducting error concealment, said first group of said one ormore of the plurality of payload bits which encode said one or morefirst spectral lines for said one or more first frequencies of thespectrum of the audio signal being smaller than or equal to thethreshold frequency, wherein said first group of said one or more of theplurality of payload bits are one or more payload bits of a redundantframe being different from the current frame.
 6. A decoder according toclaim 2, wherein, if the decoding module is configured to conduct errorconcealment in a full frame loss concealment mode, the decoding moduleis configured to conduct error concealment for all spectral lines of thespectrum.
 7. A decoder according to claim 6, wherein the plurality ofpayload bits is a plurality of current payload bits, wherein, if thedecoding module is in the partial frame loss concealment mode, thedecoding module is configured to conduct error concealment for said oneor more second spectral lines of the spectrum of the audio signal whichexhibit a frequency being greater than the threshold frequency, usingone or more stored spectral lines which have been encoded by one or moreprevious payload bits of the previous bitstream payload of the previousframe.
 8. A decoder according to claim 7, wherein the spectrum is acurrent quantized spectrum, wherein, if the decoding module isconducting error concealment in the partial frame loss concealment mode,the decoding module is configured to conduct error concealment for saidone or more second spectral lines of the spectrum of the audio signalwhich exhibit a frequency being greater than the threshold frequency, toacquire one or more intermediate spectral lines of said currentquantized spectrum, wherein the decoding module is configured to rescalethe one or more intermediate spectral lines using a rescaling factor toreconstruct the audio signal.
 9. A decoder according to claim 8, whereinthe decoding module is configured to determine the rescaling factordepending on at least one of a global gain being encoded within saidcurrent bitstream payload and a global gain being encoded within saidprevious bitstream payload, and an energy of a previous quantizedspectrum of said previous frame, an energy of a previous decodedspectrum of said previous frame, and an energy of said current quantizedspectrum of said current frame.
 10. A decoder according to claim 8,wherein the decoding module is configured to determine the rescalingfactor depending on whether or not a mean energy of spectral bins of theprevious decoded spectrum of the previous frame starting from a firstspectral bin that cannot be reconstructed without conducting errorconcealment up to a top of the spectrum is greater than or equal to amean energy of spectral bins of the previous decoded spectrum of theprevious frame starting from zero up to said last spectral bin that canbe reconstructed without conducting error concealment, or an energy ofspectral bins of said current quantized spectrum of the current framestarting from zero up to said last spectral bin that can bereconstructed without conducting error concealment is greater than orequal to an energy of spectral bins of the previous quantized spectrumof the previous frame starting from zero up to said last spectral binthat can be reconstructed without conducting error concealment.
 11. Adecoder according to claim 10, wherein, if the mean energy of thespectral bins of the previous decoded spectrum of the previous framestarting from said first spectral bin that cannot be reconstructedwithout conducting error concealment up to a top of the spectrum issmaller than the mean energy of the spectral bins of the previousdecoded spectrum of the previous frame starting from zero up to saidlast spectral bin that can be reconstructed without conducting errorconcealment, and if the energy of spectral bins of said currentquantized spectrum of the current frame starting from zero up to saidlast spectral bin that can be reconstructed without conducting errorconcealment is smaller than the energy of the spectral bins of theprevious quantized spectrum of the previous frame starting from zero upto said last spectral bin that can be reconstructed without conductingerror concealment, the decoding module is configured to determine therescaling factor such that the rescaling factor is equal to the squareroot of the ratio of the energy of the spectral bins of the currentquantized spectrum starting from zero up to said last spectral bin thatcan be reconstructed without conducting error concealment multipliedwith the square of a gain factor of the current frame, to the energy ofthe spectral bins of the previous quantized spectrum starting from zeroup said last spectral bin that can be reconstructed without conductingerror concealment multiplied with the square of a gain factor of theprevious frame.
 12. A decoder according to claim 9, wherein the decodingmodule is configured to determine the rescaling factor, being a totalrescaling factor, depends on a global gain rescaling factor, wherein thedecoding module is configured to determine the global gain rescalingfactor according to ${{fac}_{gg} = \frac{{gg}_{prev}}{gg}},$ wherein ggindicates said global gain being encoded within said current bitstreampayload, and wherein gg_(prev) indicates said global gain being encodedwithin said previous bitstream payload, and wherein fac_(gg) is theglobal gain rescaling factor.
 13. A decoder according to claim 12,wherein, $\begin{matrix}{if} & {{{\frac{1}{k_{be}}{\sum\limits_{k = 0}^{k_{be} - 1}{{\hat{X}}_{prev}(k)}^{2}}} > {\frac{1}{N_{F} - k_{be}}{\sum\limits_{k = k_{be}}^{N_{F} - 1}{{\hat{X}}_{prev}(k)}^{2}}}},{or}} \\{if} & {{{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}} > {{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}},}\end{matrix}$ the decoding module is configured to determine that thetotal rescaling factor is equal to the global gain rescaling factor,wherein k indicates a spectral bin, wherein k_(be) indicates a firstspectral bin that could not be recovered, wherein N_(F) indicates anumber of spectral lines, wherein

(k) indicates the previous quantized spectrum of the previous framebeing a last non-full frame loss concealment frame, wherein

(k) indicates the current quantized spectrum of the current frame,wherein {circumflex over (X)}_(prev)(k) indicates the previous decodedspectrum of the previous frame being said last non-full frame lossconcealment frame.
 14. A decoder according to claim 12, wherein,$\begin{matrix}{if} & {{{\frac{1}{k_{be}}{\sum\limits_{k = 0}^{k_{be} - 1}{{\hat{X}}_{prev}(k)}^{2}}} > {\frac{1}{N_{F} - k_{be}}{\sum\limits_{k = k_{be}}^{N_{F} - 1}{{\hat{X}}_{prev}(k)}^{2}}}},{and}} \\{if} & {{{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}} > {{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}},}\end{matrix}$ the decoding module is configured to determine that thetotal rescaling factor moreover depends on an energy rescaling factor${{fac}_{ener} = \sqrt{\frac{{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}}},$wherein fac_(ener) indicates the energy rescaling factor, wherein kindicates a spectral bin, wherein k_(be) indicates a first spectral binthat could not be recovered, wherein N_(F) indicates a number ofspectral lines, wherein

(k) indicates the previous quantized spectrum of the previous framebeing a last non-full frame loss concealment frame, wherein

(k) indicates the current quantized spectrum of the current frame,wherein {circumflex over (X)}_(prev)(k) indicates the previous decodedspectrum of the previous frame being said last non-full frame lossconcealment frame.
 15. A decoder according to claim 14, wherein,$\begin{matrix}{if} & {{{\frac{1}{k_{be}}{\sum\limits_{k = 0}^{k_{be} - 1}{{\hat{X}}_{prev}(k)}^{2}}} > {\frac{1}{N_{F} - k_{be}}{\sum\limits_{k = k_{be}}^{N_{F} - 1}{{\hat{X}}_{prev}(k)}^{2}}}},{and}} \\{if} & {{{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}} > {{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}},}\end{matrix}$ the decoding module is configured to determine the totalrescaling factor fac according tofac=fac _(gg) ·fac _(ener), or according to${{fac} = \sqrt{\frac{\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}{\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}},$or according to${fac} = {\frac{{gg}_{prev}}{gg} \cdot {\sqrt{\frac{{gg}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}{{gg}_{prev}^{2} \cdot {\sum\limits_{k = 0}^{k_{be} - 1}{(k)^{2}}}}}.}}$16. A decoder according to claim 1, wherein, if error concealment isconducted by the decoding module, the decoding module is configured toreconstruct a current spectrum of the audio signal by conducting errorconcealment using a plurality of signs of a previous spectrum of theaudio signal, said plurality of signs being encoded within the previousframe, wherein the decoding module is configured to conduct errorconcealment in a way that depends on whether or not said previous frameencodes a signal component which is tonal or harmonic.
 17. A decoderaccording to claim 16, wherein said previous frame is a last receivedframe, which has been decoded by the decoding module without conductingerror concealment, or wherein said previous frame is a last receivedframe, which has been decoded by the decoding module without conductingerror concealment in a full frame loss concealment mode, or wherein saidprevious frame is a last received frame, which has been decoded by thedecoding module without conducting error concealment in a partial frameloss concealment mode or in a full frame loss concealment mode.
 18. Adecoder according to claim 16, wherein, if error concealment isconducted by the decoding module, and if the previous bitstream payloadof the previous frame encodes a signal component which is tonal orharmonic, the decoding module is configured to flip one or more signs ofthe plurality of signs of the previous spectrum to reconstruct thecurrent spectrum, wherein a percentage value p, indicating a probabilityfor a sign of the plurality of signs of the previous spectrum to beflipped by the decoding module to reconstruct the current spectrum, isbetween 0%≤p≤50%, wherein the decoding module is configured to determinethe percentage value p.
 19. A decoder according to claim 18, wherein thedecoding module is configured to increase the percentage value pdepending on a number of subsequent frames; wherein said number ofsubsequent frames indicates for how many subsequently partially or fullylost frames error concealment has been conducted by the decoding module;or wherein said number of subsequent frames indicates for how manysubsequent frames error concealment in a particular error concealmentmode has been conducted by the decoding module.
 20. A decoder accordingto claim 19, wherein the decoding module is configured to determine thepercentage value p depending on a function which depends on said numberof subsequent frames, said number of subsequent frames being an argumentof said function.
 21. A decoder according to claim 20, wherein thedecoding module is configured to determine the percentage value p, suchthat p is 0%, if said number of subsequent frames is smaller than afirst threshold value; such that 0%≤p≤50%, if said number of subsequentframes is greater than or equal to the first threshold value and smallerthan a second threshold value, and such that p=50%, if said number ofsubsequent frames is greater than the second threshold value.
 22. Adecoder according to claim 21, wherein the decoding module is configuredto determine the percentage value p, such that the percentage value pincreases linearly in the range between the first threshold value andthe second threshold value depending on the number of subsequent frames.23. A decoder according to claim 18, wherein, if error concealment isconducted by the decoding module, and if the previous bitstream payloadof the previous frame does not encode a signal component which is tonalor harmonic, the decoding module is configured to flip 50% of theplurality of signs of the previous spectrum to reconstruct the currentspectrum.
 24. A decoder according to claim 1, wherein, if errorconcealment is conducted by the decoding module, the decoding module isconfigured to reconstruct a current spectrum of the audio signal byconducting error concealment using a plurality of amplitudes of theprevious spectrum of the audio signal depending on whether or not theprevious frame encodes a signal component which is tonal or harmonic,said plurality of amplitudes being encoded within the previous frame.25. A decoder according to claim 24, wherein, if error concealment isconducted by the decoding module, the decoding module is configured toattenuate the plurality of amplitudes of the previous spectrum accordingto a non-linear attenuation characteristic to reconstruct the currentspectrum, wherein the non-linear attenuation characteristic depends onwhether or not the previous frame encodes a signal component which istonal or harmonic.
 26. A decoder according to claim 24, wherein, iferror concealment is conducted by the decoding module, and if theprevious bitstream payload of the previous frame encodes a signalcomponent which is tonal or harmonic, the decoding module is configuredto attenuate the plurality of amplitudes of the previous spectrumdepending on a stability factor, wherein said stability factor indicatesa similarity between the current spectrum and the previous spectrum; orwherein the stability factor indicates a similarity between the previousspectrum and a pre-previous spectrum of a pre-previous frame precedingthe previous frame.
 27. A decoder according to claim 26, wherein saidpre-previous frame is a last received frame before the previous frame,which has been decoded by the decoding module without conducting errorconcealment, or wherein said pre-previous frame is a last received framebefore the previous frame, which has been decoded by the decoding modulewithout conducting error concealment in a full frame loss concealmentmode, or wherein said pre-previous frame is a last received frame beforethe previous frame, which has been decoded by the decoding modulewithout conducting error concealment in a partial frame loss concealmentmode or in a full frame loss concealment mode.
 28. A decoder accordingto claim 26, wherein said stability factor indicates said similaritybetween the current spectrum and the previous spectrum, if the decodingmodule is set to conduct partial frame loss concealment; wherein saidstability factor indicates said similarity between the previous spectrumand the pre-previous spectrum, if the decoding module is set to conductfull frame loss concealment.
 29. A decoder according to claim 26,wherein the decoding module is configured to determine an energy of aspectral bin of the previous spectrum; wherein the decoding module isconfigured to determine whether or not said energy of said spectral binis smaller than an energy threshold; wherein, if said energy is smallerthan said energy threshold, the decoding module is configured toattenuate an amplitude of the plurality of amplitudes being assigned tosaid spectral bin with a first fading factor, wherein, if said energy isgreater than or equal to said energy threshold, the decoding module isconfigured to attenuate said amplitude of the plurality of amplitudesbeing assigned to said spectral bin with a second fading factor, beingsmaller than the first fading factor, wherein the decoding module isconfigured to conduct attenuation such that by using a smaller fadingfactor for the attenuation of one of the plurality of amplitudes, theattenuation of said one of the amplitudes is increased.
 30. A decoderaccording to claim 26, wherein the decoding module is configured todetermine an energy of a spectral band comprising a plurality ofspectral bins of the previous spectrum; wherein the decoding module isconfigured to determine whether or not said energy of said spectral bandis smaller than an energy threshold; wherein, if said energy is smallerthan said energy threshold, the decoding module is configured toattenuate an amplitude of the plurality of amplitudes being assigned tosaid spectral bin of said spectral band with a first fading factor,wherein, if said energy is greater than or equal to said energythreshold, the decoding module is configured to attenuate said amplitudeof the plurality of amplitudes being assigned to said spectral bin ofsaid spectral band with a second fading factor, being smaller than thefirst fading factor, wherein the decoding module is configured toconduct attenuation such that by using a smaller fading factor for theattenuation of one of the plurality of amplitudes, the attenuation ofsaid one of the amplitudes is increased.
 31. A decoder according toclaim 30, wherein, if error concealment is conducted by the decodingmodule, the decoding module is configured to reconstruct a currentspectrum of the audio signal by conducting error concealment using aplurality of signs of a previous spectrum of the audio signal, saidplurality of signs being encoded within the previous frame, wherein thedecoding module is configured to conduct error concealment in a way thatdepends on whether or not said previous frame encodes a signal componentwhich is tonal or harmonic; wherein, if error concealment is conductedby the decoding module, and if the previous bitstream payload of theprevious frame encodes a signal component which is tonal or harmonic,the decoding module is configured to flip one or more signs of theplurality of signs of the previous spectrum to reconstruct the currentspectrum, wherein a percentage value p, indicating a probability for asign of the plurality of signs of the previous spectrum to be flipped bythe decoding module to reconstruct the current spectrum, is between0%≤p≤50%, wherein the decoding module is configured to determine thepercentage value p; wherein the decoding module is configured toincrease the percentage value p depending on a number of subsequentframes; wherein said number of subsequent frames indicates for how manysubsequently partially or fully lost frames error concealment has beenconducted by the decoding module; or wherein said number of subsequentframes indicates for how many subsequent frames error concealment in aparticular error concealment mode has been conducted by the decodingmodule; wherein the decoding module is configured to determine the firstfading factor such that, depending on said number of subsequent frames,the first fading factor becomes smaller, and wherein the decoding moduleis configured to determine the second fading factor such that, dependingon said number of subsequent frames, the second fading factor becomessmaller.
 32. A decoder according to claim 31, wherein the decodingmodule is configured to determine the first fading factor and the secondfading factor, so that   cum_fading_slow = 1, and cum_fading_fast = 1,

if the current frame is a first frame among the subsequent frames, andso that if the current frame is one of the frames succeeding the firstframe among the subsequent frames, the first fading factor and thesecond fading factor are determined depending on said number ofsubsequent frames according to:   cum_fading_slow = cum_fading_slow *slow; cum_fading_fast = cum_fading_fast * fast;

wherein cum_fading_slow on the right side of the formula is the firstfading factor of the previous frame, wherein cum_fading_slow on the leftside of the formula is the first fading factor of the current frame,wherein cum_fading_fast on the right side of the formula is the secondfading factor of the previous frame, wherein cum_fading_fast on the leftside of the formula is the second fading factor of the current frame,wherein 1>slow>fast>0.
 33. A decoder according to claim 32, wherein1>slow>fast>0.3.
 34. A decoder according to claim 29, wherein, if errorconcealment is conducted by the decoding module, the decoding module isconfigured to reconstruct a current spectrum of the audio signal byconducting error concealment using a plurality of signs of a previousspectrum of the audio signal, said plurality of signs being encodedwithin the previous frame, wherein the decoding module is configured toconduct error concealment in a way that depends on whether or not saidprevious frame encodes a signal component which is tonal or harmonic;wherein, if error concealment is conducted by the decoding module, andif the previous bitstream payload of the previous frame encodes a signalcomponent which is tonal or harmonic, the decoding module is configuredto flip one or more signs of the plurality of signs of the previousspectrum to reconstruct the current spectrum, wherein a percentage valuep, indicating a probability for a sign of the plurality of signs of theprevious spectrum to be flipped by the decoding module to reconstructthe current spectrum, is between 0%≤p≤50%, wherein the decoding moduleis configured to determine the percentage value p; wherein the decodingmodule is configured to increase the percentage value p depending on anumber of subsequent frames; wherein said number of subsequent framesindicates for how many subsequently partially or fully lost frames errorconcealment has been conducted by the decoding module; or wherein saidnumber of subsequent frames indicates for how many subsequent frameserror concealment in a particular error concealment mode has beenconducted by the decoding module; wherein the decoding module isconfigured to determine said energy threshold, such that said energythreshold is equal to a first energy value, if said number of subsequentframes is smaller than a third threshold value; such that said energythreshold is smaller than said first energy value and is greater than asecond energy value, if said number of subsequent frames is greater thanor equal to the third threshold value and smaller than a fourththreshold value; and such that said energy threshold is equal to saidsecond energy value, if said number of subsequent frames is greater thanthe fourth threshold value.
 35. A decoder according to claim 34, whereinthe decoding module is configured to determine the energy threshold,such that the energy threshold decreases linearly in the range betweenthe third threshold value and the fourth threshold value depending onthe number of subsequent frames.
 36. A decoder according to claim 1,wherein the decoding module comprises a decoded spectrum storage moduleconfigured for storing a decoded spectrum of the audio signal, whereinthe decoded spectrum storage module is configured to provide a lastnon-full frame loss concealment decoded spectrum, and wherein thedecoding module comprises a fade-out and sign scrambling module beingconfigured for fade-out and sign scrambling on spectral lines of thespectrum.
 37. A decoder according to claim 36, wherein the decodingmodule comprises a quantized spectrum storage module configured forstoring a quantized spectrum of the audio signal, wherein the quantizedspectrum storage module is configured to provide a last non-full frameloss concealment quantized spectrum, and wherein the decoding modulecomprises a partial frame repetition & rescaling module configured forpartial frame repetition and rescaling, wherein the partial framerepetition & rescaling module is configured to complement the spectrumby adding spectral lines, which could not be decoded by the decodingmodule, wherein the partial frame repetition & rescaling module isconfigured to re-scale said spectral lines.
 38. A decoder according toclaim 2, wherein the decoding module comprises a quantized spectrumstorage module configured for storing a quantized spectrum of the audiosignal, wherein the quantized spectrum storage module is configured toprovide a last non-full frame loss concealment quantized spectrum, andwherein the decoding module comprises a decoded spectrum storage moduleconfigured for storing a decoded spectrum of the audio signal, whereinthe decoded spectrum storage module is configured to provide a lastnon-full frame loss concealment decoded spectrum.
 39. A method fordecoding a current frame to reconstruct an audio signal, wherein theaudio signal is encoded within the current frame, wherein the currentframe comprises a current bitstream payload, wherein the currentbitstream payload comprises a plurality of payload bits, wherein theplurality of payload bits encodes a plurality of spectral lines of aspectrum of the audio signal, wherein each of the payload bits exhibitsa position within the bitstream payload, wherein the method comprises:reconstructing the audio signal, wherein, in an error concealment mode,reconstructing the audio signal is conducted by conducting errorconcealment for those spectral lines of the spectrum of the audiosignal, which exhibit a frequency being greater than a thresholdfrequency; and/or wherein, if error concealment is conducted, errorconcealment is conducted in a way that depends on whether or not aprevious bitstream payload of a previous frame preceding the currentframe encodes a signal component of the audio signal which is tonal orharmonic; and outputting the audio signal.
 40. A non-transitory digitalstorage medium having a computer program stored thereon to perform themethod for decoding a current frame to reconstruct an audio signal,wherein the audio signal is encoded within the current frame, whereinthe current frame comprises a current bitstream payload, wherein thecurrent bitstream payload comprises a plurality of payload bits, whereinthe plurality of payload bits encodes a plurality of spectral lines of aspectrum of the audio signal, wherein each of the payload bits exhibitsa position within the bitstream payload, wherein the method comprises:reconstructing the audio signal, wherein, in an error concealment mode,reconstructing the audio signal is conducted by conducting errorconcealment for those spectral lines of the spectrum of the audiosignal, which exhibit a frequency being greater than a thresholdfrequency; and/or wherein, if error concealment is conducted, errorconcealment is conducted in a way that depends on whether or not aprevious bitstream payload of a previous frame preceding the currentframe encodes a signal component of the audio signal which is tonal orharmonic; and outputting the audio signal, when said computer program isrun by a computer.